1. Field of the Invention
An image forming apparatus using an electrophotographic system normally has a fixing apparatus for melting and fixing a transferring material and toner made of a resin, magnetic material and coloring material electrostatically carried by the transferring material by heating and pressuring the transferring material and toner while nipping and conveying them by heating means (roller and endless belt body) for pressure-contacting them each other and rotating them and the pressure-contacting portion (nipping portion) of pressuring means (roller and endless belt body).
2. Related Background Art
In general, a fixing apparatus is used to thermally fix a toner image formed on a recording material serving as a recording material to the recording material in accordance with an electrophotographic recording system.
A fixing apparatus has a heating roller provided with a heater for producing heat and a pressuring roller for forming a nipping portion for pressure-contacting with the heating roller and nipping and conveying a recording material and fixes a toner image on the recording material to the recording material with heat and pressure.
A recording material to which a toner image is transferred is guided by a guide and conveyed between a hearing roller and a pressuring roller and heated and pressured and thereby, the toner on the toner image is melted and fixed to the recording material and the recording-material toner image is fixed to the recording material.
Thus, conditions necessary for fixing are a roller temperature for melting a toner layer on a recording material, time for the toner layer to pass through a heating roller and pressuring roller and pressure between the heating roller and pressuring roller and the time for the recording material to pass through the heating roller and pressuring roller is decided by circumferential speeds of the heating roller and pressuring roller and the nipping width between the heating roller and pressuring roller.
Moreover, to prevent a temperature rise of a non-sheet passing portion which occurs when passing a small-size recording material, heaters A4 and B5 are used so that calorific value distributions of the heaters A4 and B5 in the longitudinal direction become two types different from each other as shown in FIG. 5 and peak positions of these calorific values due to these calorific value distributions are not approximately overlapped. That is, in the case of a small-size recording material with a small width when forming a normal image, temperature control is performed by decreasing the turning-on rate of the heater B. Moreover, in the case of a recording material with a large width, temperature control is performed by successively turning on the both heaters in time shearing. In FIG. 5, the output of the heaters A4 is approx. 800 W and that of the heater B5 ranges from about 400 to 600 W. These values are decided so that they do not exceed the allowable power of an image forming apparatus even if the heaters A4 and B5 are simultaneously turned on under warm-up and a small-size recording material can be sufficiently fixed by only the heater A4.
In the case of a color-image forming apparatus for forming a color image by mixing a plurality of color toners, it is impossible to obtain a clear color image because an unfixed toner image expands at a nipping portion when a heating roller contacting with a toner image on a recording material has a high hardness. Therefore, by forming an elastic layer on the core bar of the heating roller and preventing the toner image from expanding by the elastic layer, it is possible to obtain a clear color image.
In the case of the starting operation of the fixing apparatus having the above configuration, it is possible to start rise of the temperature of the heating roller up to a preset start-completion temperature in a short time by fully turning on the heaters A and B with the maximum power respectively.
However, when heating a heating roller having an elastic layer, the following problem occurs. Overshoot occurs in which heat inside a heating roller is transmitted to the surface of the roller later even if a heater is turned off after the surface temperature reaches a target temperature because the heat conductivity of the elastic layer is low and time is required unit heat reaches the surface via the elastic layer after the heat is supplied to the core bar. Therefore, to decrease the overshoot, there is a method for decreasing a calorific value. In this case, however, the start time increases.
Therefore, as described in Japanese Patent Application Laid-Open NO. H62-124581, to prevent overshoot without extremely increasing the start time, there is a method for uniformly keeping a temperature distribution by forcibly turning off a main heater when the surface temperature of a heating roller reaches a predetermined temperature, operating only a subheater and causing overshoot at the central portion while only the subheater is operated.
However, an overshoot mount greatly depends on a calorific value supplied to the heating roller. That is, when turn-on time of a heater increases, the calorific value accumulated in the heating roller also increases and the overshoot amount also increases. When turn-on time of a heater decreases, the overheat decreases. Therefore, when the temperature of the heating roller is high like the case of the recovery operation after jam processing, the time for the surface temperature of the heating roller to reach a target temperature is short. Therefore, the overshoot amount decreases even if continuously turning on the main heater. Therefore, by forcibly turning off the main heater when reaching the above predetermined without variation independently of the surface temperature of the heating roller, a problem occurs that time is required for recovery because the starting operation is performed by only the subheater though the overshoot amount is small even if turning on the main heater.